The present invention relates to electrical switches and relays, and in particular, switches and relays that use an elongated shape memory alloy (SMA) element.
Shape memory alloys (SMA's) are well known alloys that are capable of undergoing plastic deformation from a "trained" shape to a "memory" shaped when heated. If the SMA material is then allowed to cool, it will deform back to its "trained" shape. The SMA material undergoes a reversible transformation from an austenitic state to a martensitic state with a change in temperature. If the SMA material is deformed to a "trained" shape while below the martensitic temperature and then heated above the austenitic temperature, the SMA material will return to its shape existing before the deformation, i.e. to its "memory" shape.
Various actuators have been proposed that use SMA's. See for example U.S. Pat. No. 4,700,541 to Kaigham J. Gabriel et al. wherein a rotary actuator is described that uses a wire made of an SMA material that is twisted or torsioned about its longitudinal axis. The ends of the wire are then constrained against movement, and a control member such as a fluid tube is bonded to the wire at a desired point. A plurality of electrical connections to the wire define different longitudinal sections of the wire to which voltages may be applied in order to heat the sections. By selectively heating and cooling the sections of the wire, the sections can work in opposition to one another in order to controllably rotate the wire and the control member.
Heretofore, SMA's have not typically been used in switches or relays. Conventional switches and relays normally are actuated by solenoid coils. These solenoid-type switches, however, can have a relatively limited reliability and can be relatively expensive to mass produce due to the assembly requirements for the solenoid.